QmlJS: Fix import path handling in the model manager.

Previously, the model manager always had exactly one import path - but
you can open more than one Qml project at once. Now, we store the union
of all import paths in the model manager.

Reviewed-by: Roberto Raggi

src/plugins/qmlprojectmanager/qmlproject.h

@@ -104,7 +104,6 @@ public:
 private slots:
     void refreshProjectFile();
     void refreshFiles(const QSet<QString> &added, const QSet<QString> &removed);
-    void refreshImportPaths();
 
 protected:
     bool fromMap(const QVariantMap &map);